Seismic activity in the Pacific Ocean near Tonga has intensified with a powerful earthquake raising fears of a tsunami.

The volcano has been erupting for several days about six miles from the southwest coast off the main island of Tongatapu, spewing spectacular plumes of white smoke and black ash out of the sea.

A 7.9-magnitude earthquake struck the area on Friday, prompting the Pacific Tsunami Warning Centre to issue a tsunami alert for Tonga and neighbouring pacific islands within 625 miles (1,000 kilometres) of the epicentre.

The warning was cancelled less than two hours later, and officials in the Tongan capital, Nuku'alofa, said there were no immediate reports of injury or damage in the 170-island archipelago.

Friday's earthquake could increase the erupting volcano's lava and ash flows, Tongan government chief seismologist Keleti Mafi said.

He said the quake will directly effect the eruption, and "could crack the volcano's vent and allow more magma to be ejected".

He added that a check of the volcano before the quake on Thursday, from a boat two miles away from the vent, showed about a 10-metre (33-foot) depth of lava standing up out of the ocean.

Coastal residents in Tonga said the steam and ash column first appeared on Monday morning, after a series of sharp earthquakes were felt in the capital, Nuku'alofa.

The underwater eruption is taking place near the low-lying twin volcanic islands of Hunga Tonga and Hunga Ha'apai, and within sight of the capital, Nuku'alofa.

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